Pulse oximeter probe

ABSTRACT

A diagnostic instrument for determining a cardiovascular system parameter. In one embodiment, the instrument takes the form of a portable pulse oximeter comprising a light to frequency converter (LFC) as a sensor. Also provided is a light to frequency converter comprising a photoresistor and capacitor in circuit communication with an inverting Schmitt trigger and configured such that the inverter generates a periodic electrical signal corresponding to the amount of electromagnetic radiation illuminating the photoresistor.

This is a continuation of application Ser. No. 08/751,645, filed Nov.18, 1996, now U.S. Pat. No. 6,011,985 which is a continuation ofapplication Ser. No. 08/221,958, filed Apr. 1, 1994, now U.S. Pat. No.5,575,284.

FIELD OF THE INVENTION

The present invention relates generally to medical diagnosticinstruments and, more specifically, to a portable pulse oximeter with aremote light-to-frequency converter as a sensor and a telemetry systemto telemeter the calculated saturation value to a remote display.

BACKGROUND OF THE INVENTION

The degree of oxygen saturation of hemoglobin, SpO₂, in arterial bloodis often a vital index of the condition of a patient. As blood is pulsedthrough the lungs by the heart action, a certain percentage of thedeoxyhemoglobin, RHb, picks up oxygen so as to become oxyhemoglobin,HbO₂. From the lungs, the blood passes through the arterial system untilit reaches the capillaries at which point a portion of the HbO₂ gives upits oxygen to support the life processes in adjacent cells.

By medical definition, the oxygen saturation level is the percentage ofHbO₂ over the total hemoglobin; therefore, SpO₂=HbO₂/(RHb+HbO₂). Thesaturation value is a very important physiological value. A healthy,conscious person will have an oxygen saturation of approximately 96 to98%. A person can lose consciousness or suffer permanent brain damage ifthat person's oxygen saturation value falls to very low levels forextended periods of time. Because of the importance of the oxygensaturation value, “Pulse oximetry has been recommended as a standard ofcare for every general anesthetic.” Kevin K. Tremper & Steven J. Barker,Pulse Oximetry, Anesthesiology, January 1989, at 98.

An oximeter determines the saturation value by analyzing the change incolor of the blood. When radiant energy passes through a liquid, certainwavelengths may be selectively absorbed by particles which are dissolvedtherein. For a given path length that the light traverses through theliquid, Beer's law (the Beer-Lambert or Bouguer-Beer relation) indicatesthat the relative reduction in radiation power (P/Po) at a givenwavelength is an inverse logarithmic function of the concentration ofthe solute in the liquid that absorbs that wavelength.

For a solution of oxygenated human hemoglobin, the absorption maximum isat a wavelength of about 640 nanometers (red), therefore, instrumentsthat measure absorption at this wavelength are capable of deliveringclinically useful information as to oxyhemoglobin levels.

In general, methods for noninvasively measuring oxygen saturation inarterial blood utilize the relative difference between theelectromagnetic radiation absorption coefficient of deoxyhemoglobin,RHb, and that of oxyhemoglobin, HbO₂. The electromagnetic radiationabsorption coefficients of RHb and HbO₂ are characteristically tied tothe wavelength of the electromagnetic radiation traveling through them.

It is well known that deoxyhemoglobin molecules absorb more red lightthan oxyhemoglobin molecules, and that absorption of infraredelectromagnetic radiation is not affected by the presence of oxygen inthe hemoglobin molecules. Thus, both RHb and HbO₂ absorb electromagneticradiation having a wavelength in the infrared (IR) region toapproximately the same degree; however, in the visible region, the lightabsorption coefficient for RHb is quite different from the lightabsorption coefficient of HbO₂ because HbO₂ absorbs significantly morelight in the visible spectrum than RHb.

In practice of the pulse oximetry technique, the oxygen saturation ofhemoglobin in intravascular blood is determined by (1) alternativelyilluminating a volume of intravascular blood with electromagneticradiation of two or more selected wavelengths, e.g., a red wavelengthand an infrared wavelength, (2) detecting the time-varyingelectromagnetic radiation intensity transmitted through or reflectedback by the intravascular blood for each of the wavelengths, and (3)calculating oxygen saturation values for the patient's blood by applyingthe Lambert-Beer's transmittance law to the detected transmitted orreflected electromagnetic radiation intensities at the selectedwavelengths.

Whereas apparatus is available for making accurate measurements on asample of blood in a cuvette, it is not always possible or desirable towithdraw blood from a patient, and it obviously impracticable to do sowhen continuous monitoring is required, such as while the patient is insurgery. Therefore, much effort has been expanded in devising aninstrument for making the measurement by noninvasive means.

The pulse oximeters used today are desk-top models or handheld modelsthat are interfaced to the patient through the use of a multi-wirebundle. Despite their size and level of technology, these units arestill bound by several limitations.

A critical limitation is that of measurement accuracy. In pulseoximetry, signal artifact from patient-probe motion, ambient light, andlow perfusion (low blood circulation through the extremities) is one ofthe primary causes of inaccurate saturation readings. (“Artifact” is anycomponent of a signal that is extraneous to variable represented by thesignal.) Inaccuracies are also caused from physiologic nonlinearitiesand the heuristic methods used to arrive at the final saturation values.

Another important limitation is patient confinement to the pulseoximeter, due to the wired probe connecting the patient to the unit.This limits patient mobility in every application of its use, includingthe emergency room, operating room, intensive care unit, and patientward.

Thus, three problems plague pulse oximetry. The first problem relates tosignal artifact management and inaccuracies of the saturation values dueto the non-linear nature of the sample tissue bed. The second problemrelates to noise from signal artifact which introduces furtherinaccuracies. The third problem relates to restricted patient mobilityand probe placement due to the wire bundle that physically couples thepatient to the oximeter unit and the exclusive use of transmittance-typeprobes.

Due to the non-linear nature of human physiology, engineers were forcedto employ techniques for calculating the final saturation value basednot on an analytic solution, but rather, on a calibration curve orlook-up table derived from empirical data. This is data that has beencollected over hundreds or possibly thousands of patients and stored asa look-up table in the system memory. This technique leads to obviousinaccuracies in the final saturation value since the SpO₂ value in thelook-up table is only as accurate as the calibration curve programmedinto the system memory, which in turn is only as accurate as the invitro laboratory oximeter used to generate it. These inaccuracies arecompounded by differences in skin characteristics between patients, aswell as differences over the skin surface of the same patient.

Signal artifact has three major sources: (1) ambient light (which causesan AC/DC masking signal), (2) low perfusion (in which the intensity ofthe desired AC/DC signal is very low thereby allowing other artifactsources to mask the desired signal more easily), and (3) patient orsensor motion (which generates a large AC/DC artifact masking thedesired signal). When the oximetry signal is amplified, the noisecomponents are amplified along with the desired signal. This noise actsto corrupt the primary signal, during both pre-processing as well aspost-processing, thereby reducing the accuracy of the pulse oximeterreading. Signal artifact is prevalent with both reflectance- andtransmittance-type probes.

Restricted patient mobility is due to the hard wired interface thatlinks the patient probe to the large, bulky oximeter unit. This link isa multi-wire bundle that is used to provide an electrical path for theLED drivers and the photodiode located at the end of the wire bundle inthe probe. Probes employing transmittance-type method are restricted tothe ears, fingers, or toes and, thus, require physical access to theseareas exclusively.

Oximeters are large because of the circuitry heretofore believednecessary to capture the signals and because such higher-poweredcircuitry shortens battery life. Typical digital oximeters use a siliconphotodiode, a current-to-voltage converter (a transimpedance amplifier),a preamplifier, filter stage, a sample and hold, and ananalog-to-digital (A/D) converter to capture the oximetry signal. Thesecomponents make the creation of truly portable oximeters difficultbecause of the large footprint and high power requirements of eachdevice. The A/D converter, in particular, is typically large andpower-hungry.

SUMMARY OF THE INVENTION

According to the present invention, an oximeter is provided with alight-to-frequency converter as a sensor and a telemetry system totelemeter the calculated saturation value to a remote station. Thelight-to-frequency converter eliminates the need for a separatephotodiode, a current-to-voltage converter, a preamplifier, a filter, asample and hold, and an analog-to-digital (A/D) converter found intypical digital oximeters, thereby significantly reducing the circuitfootprint and power consumption. In short, the light-to-frequencyconverter can be directly connected to an input of a microcontroller orother CPU. The use of telemetry allows accurate hemoglobin saturationlevel determination to be made without the patient being tethered by awire bundle to a remote display. Powerful portable systems can berealized using very large-scale integrated circuit (VLSI) multichipmodule (MCM) technology.

An oximeter made under the present invention is a truly portable unit,capable of capturing and processing oximetry data in a very smallpackage and transmitting calculated saturation values to a remotereceiver. The type of receiver that is particularly useful in thecontext of the present invention is a caregiver's wrist receiver orother type of receiver that communicates to a primary caregiver. Inaddition, this invention can communicate with other types of receivers,such as a nurses' station receiver or some other personal data receiver.Spread spectrum communication techniques allow highly secure andnoise-immune telemetry of saturation values in noisy clinical andhealthcare environments.

The oximeter of the present invention uses a pair of light emittingdiodes, a light-to-frequency converter, a high-speed counter, a computersystem, and an display or other output.

According to the present invention, two light emitting diodes (LEDs), ared LED and an infrared LED, alternatively illuminate an intravascularblood sample with two wavelengths of electromagnetic radiation. Theelectromagnetic radiation interacts with the blood and a residualoptical signal is both reflected and transmitted by the blood. Aphotodiode in the light-to-frequency converter (LFC) collects oximetrydata from the intravascular blood sample illuminated by the two LEDs.The LFC produces a periodic electrical signal in the form of a pulsetrain having a frequency, the logarithm of which is in linearrelationship to the logarithm of the intensity of the optical signalreceived by the LFC. The data becomes an input to a high-speed digitalcounter, which converts the pulsatile signal into a form suitable to beentered into a central processing unit (CPU) of a computer system.

In the alternative, a CPU with an internal counter can be used, therebyeliminating the need for an external counter and further reducing thesystem size.

Once inside the CPU, the time-domain data is converted into thefrequency domain by, for example, performing the well-known Fast FourierTransform (FFT) on the time-domain data. The frequency domain data isthen processed to determine the saturation value.

It is therefore an advantage of the present invention to provide aportable, low-power oximeter.

It is a further object of this invention to provide an improved sensorin the form of a light-to-frequency converter to reduce the parts countof prior art systems.

These and other advantages of the present invention shall become moreapparent from a detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, which are incorporated in and constitute apart of this specification, embodiments of the invention areillustrated, which, together with a general description of the inventiongiven above, and the detailed description given below serve to examplethe principles of this invention.

FIG. 1 is an electrical schematic representation of a generic prior artpulse oximeter;

FIG. 2A is an electrical schematic representation of one embodiment of apulse oximeter of the present invention;

FIG. 2B is an electrical schematic representation of another embodimentof a pulse oximeter of the present invention;

FIG. 3A is an electrical schematic representation of the implementationof the TSL220 light-to-frequency converter in the oximeter of thepresent invention;

FIG. 3B is an electrical schematic representation of the implementationof the TSL230 light-to-frequency converter in the oximeter of thepresent invention;

FIG. 4A is an electrical schematic representation of an implementationof a light-to-frequency converter of the present invention;

FIG. 4B is another embodiment of the LFC shown in FIG. 4A;

FIG. 4C is yet another embodiment of the LFC shown in FIG. 4A; and

FIG. 4D is still another embodiment of the LFC shown in FIG. 4A; and

FIG. 5 is a flow chart showing the major process steps taken by thecomputer system in calculating the saturation value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Before describing the details of the present invention, a description ofa generic prior art pulse oximeter may be helpful in understanding theadvantages of the pulse oximeter of the present invention. Reference ishad, therefore, to FIG. 1, which shows a generic prior art pulseoximeter 10. A typical prior art oximeter 10 has a photodiode 12 fordetecting an optical signal 14 reflected from or transmitted through avolume of intravascular blood (not shown) illuminated by one or morelight emitting diodes (LEDs, not shown). The LEDs emit electromagneticradiation at a constant intensity; however, an optical signal 14 with atime-varying intensity is transmitted through or reflected back from theintravascular blood for each of the wavelengths. The photodiode 12generates a low-level current proportional to the intensity of theelectromagnetic radiation received by the photodiode 12. The current isconverted to a voltage by a current to voltage converter 16, which maybe an operational amplifier in a current to voltage (transimpedance)configuration.

The signal is then filtered with a filter stage 18 to remove unwantedfrequency components, such as any 60 Hz noise generated by fluorescentlighting. The filtered signal is then amplified with an amplifier 20 andthe amplified signal is sampled and held by a sample and hold 21 whilethe signal is digitized with a high-resolution (12-bit or higher) analogto digital converter (ADC) 22.

The digitized signal is then latched by the CPU (not shown) of thecomputer system 24 from the ADC 22. The computer system 24 thencalculates a coefficient for the oxygen saturation value from thedigitized signal and determines the final saturation value by readingthe saturation value for the calculated coefficient from a look-up tablestored in memory. The final saturation value is displayed on a display26.

Thus, the generic prior art pulse oximeter 10 requires numerous devicesto determine the oxygen saturation value from the optical signal.Moreover, these devices, particularly the ADC 22, require a relativelylarge amount of space and electrical power, thereby rendering a portableunit impractical.

Under the present invention, the prior art oximeter 10 is modified sothat the photodiode 12, current to voltage converter 16, filter 18,amplifier 20, sample and hold 21, and analog-to-voltage converter 22 arereplaced with a light-to-frequency converter and a high speed counter.

FIG. 2A shows one embodiment of a pulse oximeter 50 of the presentinvention. The oximeter 50 of the present invention comprises alight-to-frequency converter (LFC) 52 for detecting an optical signal 54from a volume of intravascular volume of blood 56 illuminated by one ormore light emitting diodes (LEDs) 58, 60. The LEDs 58, 60 emitelectromagnetic radiation at a constant intensity; however, an opticalsignal 54 with a time-varying intensity is transmitted through orreflected back by the intravascular blood for each of the wavelengths.In the preferred embodiment, the reflected optical signal 54 is analyzedto determine the saturation value. The LFC 52 produces a periodicelectrical signal in the form of a pulse train having a frequencycorresponding to the intensity of the broadband optical signal receivedby the LFC 52. The periodic data then becomes an input to a high-speeddigital counter 62, which converts the periodic signal into a formsuitable to be entered into a computer system 64.

Once inside the computer system 64, the LFC signal is analyzed todetermine the saturation value. In one embodiment, the data is convertedinto the frequency domain by, for example, performing the well-knownFast Fourier Transform (FFT) on the data. It is also believed that othercommon techniques of converting time-domain data to the frequency domainwill suffice: e.g., discrete cosine transform, wavelet transform,discrete Hartley transform, and Gabor transform. The frequency domaindata is then analyzed to determine the saturation value by codeexecuting on the computer system 64, as will be more fully explained inthe text accompanying FIG. 4. Once calculated, the saturation value isdisplayed on a display 68.

In addition to performing saturations calculations, the computer system64 controls LED drivers 66, which control the LEDs 58, 60.

FIG. 2B shows another embodiment of the pulse oximeter of the presentinvention. The embodiment of FIG. 2B differs from the embodiment in FIG.2A in two respects. First, the computer system 64 and counter 62 areimplemented by a microcontroller 84 having an internal high-speedcounter 82 associated therewith. Second, the microcontroller 84 and thedisplay 68 are placed in circuit communication using a transmitter 86and receiver 88. The transmitter 86 transmits a signal 90 through anantenna 92. The receiver 88 receives the signal 90 through a secondantenna 94 and passes the information to the display circuit 68.

The LFC 52, the counter 62, the computer system 64, the display 68, theLED drivers 66, the LEDs 58, 60, and the other components are connectedin electrical circuit communication as shown in FIGS. 2A and 2B. Onesuitable LFC 52 is the TSL220, manufactured and sold by TexasInstruments, P.O. Box 655303, Dallas, Tex. 75265. FIG. 3A is anelectrical schematic representation showing the use of the TSL220 in theoximeter of the present invention. The capacitor 70 and resistor 72 arein circuit communication and have the values as shown in that figure.Another suitable LFC 52 is the TSL230, shown in FIG. 3B, is manufacturedby Texas Instruments. Unlike the TSL220, the TSL230 requires no externalcapacitor and provides microprocessor compatible control lines;therefore, the TSL230 is a one-chip sensor.

Yet another suitable LFC 52 is a novel LFC circuit, which was inventedby Stephan Peter Athan, one of the coinventors of this invention, and isshown in FIG. 4A. In that circuit, a photoresistor 73 having a variableresistance is placed in circuit communication with a pulse generatingcircuit that is configured to generate a periodic electrical signalcorresponding to the value of the variable resistance of thephotoresistor. In one embodiment, a photoresistor 73, a capacitor 74,and an inverter 75 are placed in circuit communication and have thevalues shown in that figure. The photoresistor 73 is placed across theinput node 76 and the output node 77 of the inverter 73. The capacitor74 is placed between the input node 76 and ground. The inverter 75 isideally an inverting Schmitt trigger with hysteresis at its input;however, other inverters are also believed to be suitable.

The photoresistor 73 can be a standard cadmium sulfide or cadmiumselenide photoresistor, which are both widely available from manysources. Other types of photoresistors are also available. As is knownin the art, the photoresistor 73 has a variable resistance that dependson the amount of electromagnetic radiation 78 being emitted onto thephotoresistor. The photoresistor 73, capacitor 74, and inverter 75 areconfigured such that the period of time in which the capacitor 74charges and discharges corresponds to the value of the variableresistance of the photoresistor 73. Thus, the output of the inverter 75is a periodic signal, the period of which depends on the amount ofelectromagnetic radiation being emitted onto the photoresistor 73.

As shown in FIG. 4B, a resistor 79 with a substantially fixed resistancecan be placed in series with the photoresistor 78 and placed across theinput 76 and output 77 of the inverter 75. In addition, as shown in FIG.4C, a multiplying digital to analog converter (MDAC) 80 can be placed inseries with the photoresistor 73 and placed across the input 76 and theoutput 77 of the inverter 75. As shown in that figure, the MDAC 80 isinterfaced to the microcontroller 84, which can then control theparameters, and therefore the sensitivity (i.e., shifting the frequencyassociated with a given amount of illumination to accommodate a broaderrange of light frequencies), of the circuit by selectively assertingmore or less resistance in series with the photoresistor 73. Onesuitable MDAC is the AD7524 available from Analog Devices, which isessentially a computer controlled R2R network, which is known in theart.

As shown in FIG. 4D, a bank of capacitors with varying capacitancevalues can be connected in the circuit of FIG. 4A. The capacitors areinterfaced to the circuit via a computer controlled bank of analogswitches, as shown in that figure. The microcontroller 84 can controlthe parameters of the circuit, and therefore the sensitivity (i.e.,shifting the frequency associated with a given amount of illumination toaccommodate a broader range of light frequencies), by selectivelyconnecting one or more of the capacitors to line 76.

While the LFC of FIGS. 4A-4D is believed to be particularly useful inconnection with the portable pulse oximeter of the present invention, itis also believed to have utility beyond that of oximetry or othercardiovascular measurement.

Referring back to FIGS. 2A and 2B, the Red LED 58 is a red LED, emittinglight having a wavelength of approximately 660 nm. One suitable LED isthe P417-ND, which is available from by Digikey, 701 Brooks AvenueSouth, Thief River Falls, Minn. 56701. It is believed that an LEDemitting any wavelength of light in the visible spectrum is suitable;however, because a solution of human hemoglobin has an absorptionmaximum at a wavelength of about 640 nanometers (red), the closer tothat wavelength, the more accurate the results (otherwise, calibrationcurves are required, as is known in the art).

The IR LED 60 is an infrared LED, emitting electromagnetic radiationhaving a wavelength of approximately 940 nm. One suitable LED is theF5F1QT-ND, which is also available from Digikey. It is believed that tobe suitable, the IR LED 60 must emit electromagnetic radiation at awavelength such that the absorption of the emitted electromagneticradiation by the blood 56 is unaffected by the presence or absence ofoxygen bound to the hemoglobin molecules.

The counter 62 may be any high speed counter capable of being interfacedto a computer system. One suitable counter is the 4020 CMOS counter,which is manufactured by numerous manufacturers, e.g., TexasInstruments, P.O. Box 655303, Dallas, Tex. 75265, as is well known inthe art.

Interfacing the counter 62 to the computer system 64 may be done inseveral ways. The counter 62 and computer system 64 may be configured toeither (1) count the pulses generated by the LFC 52 during a given timeperiod or (2) count the number of pulses of a free-running clock(corresponding to the amount of time) between the individual pulses ofthe LFC 52. Either method will provide satisfactory data. The lattermethod can be implemented in several ways. For example, the counter canbe reset at each period of the LFC signal. In the alternative, at eachedge of LFC pulse train, the value in the counter can be saved to aregister and subtracted from the value stored at the previous edge.Either way, the result is a counter value corresponding to the timedifference between the two pulse edges. Many configurations arepossible. The counter 62 can either count pulses or elapsed time betweenedges and the computer system 64 either reads the value in the counterperiodically by polling the counter, or the computer system 64 reads thevalue whenever the counter 62 generates an interrupt. Again, manyconfigurations are possible.

The computer system 64 can be any computer system capable of performingoximetry calculations to the desired accuracy in the desired period oftime (calculations may be done either in real time or after collectionof desired data) and capable of interfacing with a counter 62, a display68, and LED drivers 66. The computer system 64 may include a CPU, randomaccess memory (RAM), read-only memory (ROM), and associated controlcircuitry, such as decoders and multi-phase clocks, in circuitcommunication, as is well known in the art. To be suitable, the computersystem must be capable of being a signal analyzer. That is, the computersystem 64 must have the computational capacity to determine thesaturation value from the periodic pulses.

One suitable computer system 64 is any of several microcontrollers 84,which are known in the art. The 68HC16 microcontroller manufactured byMotorola, Inc., Austin Tex. 78735, is one example. The 68HC16 issuitable for systems requiring low-level digital signal processing andhas on-board erasable/programmable ROM (EPROM) and RAM. It also has anon-board 16-bit high-speed counter 82 eliminating the need for anexternal counter 62. The output from the LFC 52 may be directlyconnected to the counter input of the 68HC16, thereby allowing theelimination of another discrete device (the separate counter 62).Another suitable microcontroller 84 is the 80CX51FA, which ismanufactured by Intel Corp., Santa Clara, Calif. 95051.

If more processing power than either the 68HC16 or the 80CX51FA canprovide is, required to determine the saturation value, a digital signalprocessor or floating point coprocessor (not shown) can be added to thecomputer system 64. One suitable digital signal processor is theTMS320CX0 digital signal processor, manufactured by Texas Instruments.This device can calculate highly accurate oxygen saturation values in aperiod of time on the order of microseconds.

The LED drivers 66 may be any driver capable of providing a signalcapable of causing one or more LEDs to illuminate. Numerous LED-drivingcircuits are well known in the art. The drivers 66 must allow the LEDs58, 60 to be alternatively illuminated under control of the computersystem 64.

Some prior art LED drivers have a normalizing function that increases ordecreases the intensity of electromagnetic radiation generated by theLEDs in the system. It is desirable to be able use a single oximeterconfiguration to measure the oxygen saturation of an infant and later touse the same oximeter configuration to measure oxygen saturation levelsof an adult. Since the nature of skin and hair of an infant aredifferent from that of an adult, it is generally accepted that an LEDintensity calibrated to measure the oxygen saturation level of an adultwill be too bright to measure the oxygen saturation level of an infant(the optical signal 54 is so bright that the photodiode saturates).Likewise, it is generally accepted that an LED intensity calibrated tomeasure saturation of an infant will be too dim to provide adequate datato measure the oxygen saturation of an adult. The normalizing functionin some prior art oximeters adjusts the intensities of the LEDs toprovide a useful signal under most circumstances.

In the oximeter 50 of the present invention, the normalizing function isnot needed. The TSL220 and TSL230 both have 118 Db dynamic ranges.Moreover, the TSL230 has a computer-interfacable gain control foramplification or attenuation of the optical signal, thereby providing aneven higher dynamic range. In addition, the LFC of FIG. 4A has a muchwider dynamic range. These very wide dynamic ranges allow the use of LEDdrivers to be configured such that the intensities of the LEDs 58, 60are set at fixed, predetermined values. The LFCs 52 are so sensitivethat an LED intensity suitable for an infant will still generate areflected optical signal 54 in an adult strong enough to determine thesaturation value of that adult. Thus, the LED drivers 66 need not havethe ability to normalize the intensities of the LEDs 58, 60.

The display 68 can be any display capable of displaying one or moreoxygen saturation values to the desired resolution. Well known displaysinclude seven segment LED displays and liquid crystal displays (LCDs),all of which are well known in the art. In addition, discrete LEDs maybe used if the designer desires to display merely a binary oxygensaturation level. For example, green, yellow, and red discrete LEDs canbe configured to represent normal, warning, and critical conditionscorresponding to saturation values of greater than 90 percent, 80 to 90percent, and less than 80 percent, respectively.

The transmitter 86, receiver 88, and the two antennas 92, 94 can be anysuitable radio frequency or other wireless telemetry system. Thesetelemetry systems are well known in the art and widely available.Additionally, spread spectrum technology provides a highly secure link,a high noise immunity, and a high informational capacity, all of whichare desirable in clinical and healthcare environments. A suitable spreadspectrum transmitter/receiver pair is believed to be available fromProxim, Mountain View, Calif.

The use of the LFC 52 allows the design of a truly portable pulseoximeter. The TSL230 and the 68HC16 are available in die form (themonolithic electronic device without external packaging or leads),allowing a multichip model (MCM) to be fabricated by connecting thedevices at the die-level, as is well known in the art. Displaytechnology for wrist watches and other small devices, also well known inthe art, provides a very compact and low-power display 68. LED driverscan comprise surface-mount 2N2222 NPN transistors and surface-mountresistors, both of which are well known in the art and available fromnumerous sources. Thus, an extremely small pulse oximeter can beconstructed according to the principles of the present invention.

Such a pulse oximeter can be light and small enough to be worn by anambulatory patient. That is, the oximeter can be made light enough andotherwise configured to be worn by a patient in the manner that a wristwatch, bracelet, anklet, inflatable cuff, etc. might be worn.

In addition, telemetry circuits are also available as portable systemsfrom Proxim, Mountain View, Calif., allowing an oximeter to telemeterfinal oximetry data to the receiver, such as a caregiver's wristreceiver or other type of receiver that communicates the saturationvalue to a primary caregiver.

In constructing a pulse oximeter according to the present invention, theLEDs 58, 60, the light to frequency converter 52, the counter 62, themicroprocessor system 64, and either the display 68 or the transmitter86 are preferably packaged within a single package. For example, theLEDs 58, 60, LFC 52, microcontroller 84 with internal counter 82, andtransmitter 86 can be packaged in a small package about the size andconfiguration of a wrist watch, with the LEDs 58, 60 and LFC 52 placedin optical communication with the patient's skin. In this example, theantenna 92 can be positioned in a band similar to that of a wrist watch,which can wrap around or otherwise encircle a part of a patient'sanatomy (arm, ankle, neck, etc.) to secure the package to the patient,or the antenna 92 can be positioned externally of the package. Asanother example, the LEDs 58, 60, LFC 52, microcontroller 84 withinternal counter 82, and display 68 can be packaged in an inflatablecuff system with the LEDs 58, 60 and LFC 52 placed in opticalcommunication with the patient's skin and the display 68 positioned tobe readable by a caregiver. The inflatable cuff can be any of thoseknown in the art, such as those used with blood pressure monitoringequipment.

Virtually any digital pulse oximeter design could be modified to use theLFC 52 of the present invention. Therefore, the invention in its broaderaspects is not limited to the specific details, representative apparatusand method, and illustrative examples shown and described. Accordingly,departures may be made from such details without departing from thespirit or scope of the applicant's general inventive concept.

FIG. 5 is a flow chart showing the role of the computer system 64 in thepulse oximeter 50 of the present invention.

First, the computer system 64 initializes the system, at 100. Suchinitialization is very system-specific and is well known in the art.After initializing the system, the computer system 64 begins collectingsamples of data. A “sample” is the reading of two intensity values withthe LCF 52 and counter 62: (1) an intensity value with the red LED 58constantly emitting and the infrared LED 60 not emitting and (2) anintensity value with the IR LED 60 constantly emitting and the red LED58 not emitting.

When either of the LEDs 58, 60 is emitting and a signal 54 is beinggenerated by the interaction of the electromagnetic radiation with theblood 56, the LFC 52 generates a periodic electrical signal in the formof a pulse train with a period corresponding to the intensity of theoptical signal 54 received by the LFC 52. This signal is interfaced intothe computer system 64 with the counter, as described above, and anintensity value for the red LED 58 and an intensity value for the IR LED60 are saved in RAM.

Data collection begins at 102. The total collection period is 4.27seconds in this embodiment, which is divided into four quarters ofapproximately one second each. As shown at 102, three quarters(approximately three seconds) of data samples are collected to helpinitialize a sliding window function, described below. Next, the fourthquarter of the total sample (approximately one second worth of samples)is taken, at 104. The sample rate and time of collection are allvariable. In this embodiment, between the samples taken at 102 and 104,a total of 4.27 seconds worth of samples are collected for processing.The samples can be taken at many rates, e.g., 15 hertz to 240 Hz,depending on the processing to take place below, as is known in the art.

The system then determines the magnitudes of the AC and DC componentsfor both the Red LED 58 and the IR LED 60 (AC_(red), DC_(red), AC_(ir),and DC_(ir)) using a frequency domain analysis, at 106. That is, the4.27 seconds of time-domain data is then converted into the frequencydomain by performing the well-known Fast Fourier Transform (FFT). TheFFT can be performed in many ways, as is known in the art. For example,an FFT of between 64 points (on data sampled at 15 Hz) and 1024 points(on data sampled at 240 Hz) will suffice. For both the red and IRsignals, the AC component is determined by the magnitude of the highestspectral peak found at from 0.5 to 2.5 Hz and represents the pulsatile,or AC component, of the oximetry waveform. Likewise, the magnitude ofthe DC component is the highest spectral peak found at from between 0.0and 0.5 Hz.

Next, at 108, the program calculates an R value from the red andinfrared AC and DC spectral peaks, based on the formula:R=(AC_(red)/DC_(red))/(AC_(ir)/DC_(ir)) Finally, an SpO₂ value isobtained from the approximate formula:

SpO₂=−25R+110.

These steps were verified using Matlab, which is available from TheMathworks, Inc., Natick, Mass. The formulae applied to the spectral dataallowed satisfactory oxygen saturation levels to be calculated. Thevalues were to within 2% (one standard deviation) of correct oxygensaturation levels, as measured by an IL-282 Cooximeter for the waveformsanalyzed, which is adequate for commercial use.

As is known, in the alternative to the FFT, many other methods can beused to determine the AC and DC components. For example, the well knowndiscrete cosine transform, wavelet transform, discrete Hartleytransform, and Gabor transform can all be used.

Next, the calculated saturation value is displayed on the display 68, asis well known in the art, at 110.

Finally, the program loops back to 104, where another one quarter of4.27 seconds of data is collected. As indicated at 112, the oldestquarter of data is discarded so that 4.27 seconds of data remain (onlyapproximately one second of which is new). Thus a 4.27 second window ofdata can be thought of as sliding by one-quarter increments, therebydiscarding approximately one second of data and sampling a new onesecond of data. The steps at 104, 106, 108, 110, and 112 are performedrepeatedly, thereby displaying a new SpO₂ value approximately eachsecond.

While the present invention has been illustrated by the description ofembodiments thereof, and while the embodiments have been described inconsiderable detail, it is not the intention of the applicant torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. For example, other light-to-frequencyconverters can be used. As another example, a 2048-point FFT can beperformed on 8.53 seconds of data collected at 240 Hz. Finally, withminor modifications to the signal analysis portion of this system, thepresent invention can be used as a diagnostic instrument for determiningother cardiovascular system parameters, such as pulse rate, respirationrate, and intravascular volume status. Therefore, the invention in itsbroader aspects is not limited to the specific details, representativeapparatus and method, and illustrative examples shown and described.Accordingly, departures may be made from such details without departingfrom the spirit or scope of the applicant's general inventive concept.

We claim:
 1. A diagnostic probe for measuring a parameter of acardiovascular system comprising: an electromagnetic radiation sourcefor illumination of a volume of intravascular blood with electromagneticradiation, the electromagnetic radiation having at least one frequencycomponent having an intensity, the intensity of the at least onefrequency component of the electromagnetic radiation being altered byinteraction with the volume of intravascular blood thereby generating anoptical signal from the interaction of the at least one frequencycomponent of the electromagnetic radiation with the volume ofintravascular blood, the optical signal having an intensity; and alight-to-frequency converter having an electromagnetic radiation sensorelectrically connected to an electronic signal generator, saidelectromagnetic radiation sensor being optically coupled to saidelectromagnetic radiation source for reception of the optical signal andsaid electronic signal generator generating a periodic electrical signalresponsive to said electromagnetic radiation sensor, the periodicelectrical signal comprising a digital pulse train having pulses, thetiming of the pulses of the electrical signal corresponding to theintensity of the optical signal.
 2. A diagnostic probe according toclaim 1 further comprising an inflatable cuff associated with saidelectromagnetic radiation source and wherein said electromagneticradiation source is placed in optical communication with a patient'sskin associated with the volume of intravascular blood.
 3. A pulseoximeter probe comprising: an electromagnetic radiation source forillumination of a volume of intravascular blood with electromagneticradiation, the volume of intravascular blood having an oxygen saturationlevel of hemoglobin within the volume of blood, the electromagneticradiation having at least one frequency component having an intensity,the intensity of the at least one frequency component of theelectromagnetic radiation being altered by interaction withoxyhemoglobin in the volume of intravascular blood thereby generating anoptical signal from the interaction of the at least one frequencycomponent of the electromagnetic radiation with the oxyhemoglobin in thevolume of intravascular blood, the optical signal having an intensity;and a light-to-frequency converter having an electromagnetic radiationsensor electrically connected to an electronic signal generator, saidelectromagnetic radiation sensor being optically coupled to saidelectromagnetic radiation source for reception of the optical signal andsaid electronic signal generator generating a periodic electrical signalresponsive to said electromagnetic radiation sensor, the periodicelectrical signal comprising a digital pulse train having pulses, thetiming of the pulses of the electrical signal corresponding to theintensity of the optical signal.
 4. A pulse oximeter probe according toclaim 3 wherein a value associated with the intensity of the opticalsignal received by said electromagnetic radiation sensor has alogarithm, the periodic electrical signal generated by said electronicsignal generator of said light-to-frequency converter has a frequency, avalue associated with the frequency has a logarithm, and the logarithmof the value associated with the frequency is in linear relationship tothe logarithm of the value associated with the intensity of the opticalsignal received by said electromagnetic radiation sensor of saidlight-to-frequency converter.
 5. A pulse oximeter probe according toclaim 3 wherein said electromagnetic radiation source comprises a firstemitter of electromagnetic radiation for selectively illuminating theintravascular blood with electromagnetic radiation of a first frequencyand a second emitter of electromagnetic radiation for selectivelyilluminating the intravascular blood with electromagnetic radiation of asecond frequency; wherein electromagnetic radiation at said secondfrequency has an absorption coefficient with respect to oxyhemoglobin;and wherein electromagnetic radiation at said first frequency has anabsorption coefficient with respect to oxyhemoglobin that issubstantially different than the absorption coefficient with respect tohemoglobin of electromagnetic radiation at said second frequency.
 6. Apulse oximeter according to claim 5 further comprising an inflatablecuff associated with said electromagnetic radiation source and whereinsaid electromagnetic radiation source is placed in optical communicationwith a patient's skin associated with the volume of intravascular blood.7. A pulse oximeter according to claim 3 further comprising aninflatable cuff associated with said electromagnetic radiation sourceand wherein said electromagnetic radiation source is placed in opticalcommunication with a patient's skin associated with the volume ofintravascular blood.
 8. A diagnostic instrument probe for determining aparameter of a cardiovascular system comprising: an electromagneticradiation source for illumination of a volume of intravascular bloodwith electromagnetic radiation, the electromagnetic radiation having atleast one frequency component having an intensity, the intensity of theat least one frequency component of the electromagnetic radiation beingaltered by interaction with the volume of intravascular blood therebygenerating an optical signal from the interaction of the at least onefrequency component of the electromagnetic radiation with the volume ofintravascular blood, the optical signal having an intensity; and alight-to-frequency converter having an electromagnetic radiation sensorelectrically connected to an electronic signal generator, saidelectromagnetic radiation sensor being optically coupled to saidelectromagnetic radiation source for reception of the optical signal andsaid electronic signal generator generating a periodic electrical signalresponsive to said electromagnetic radiation sensor, the periodicelectrical signal comprising a digital pulse train having pulses, thetiming of the pulses of the electrical signal corresponding to theintensity of the optical signal.
 9. A diagnostic instrument probeaccording to claim 8 wherein said inverting circuit has an input and anoutput and wherein said photoresistor is electrically connected in afeedback path between said output and said input of said invertingcircuit.
 10. A diagnostic instrument probe according to claim 8 furthercomprising an additional resistive device, wherein said invertingcircuit has an input and an output, and wherein said photoresistor iselectrically connected in series relation with the additional resistivedevice in a feedback path between said output and said input of saidinverting circuit.
 11. A diagnostic instrument probe according to claim8 wherein said inverting circuit has an input and an output and whereinthe digital pulse train of the periodic electrical signal is generatedat said output of said inverting circuit.
 12. A diagnostic instrumentprobe according to claim 8 further comprising an inflatable cuffassociated with said electromagnetic radiation source and wherein saidelectromagnetic radiation source is placed in optical communication witha patients skin associated with the volume of intravascular blood.
 13. Apulse oximeter probe comprising: an electromagnetic radiation source forillumination of a volume of intravascular blood with electromagneticradiation, the volume of intravascular blood having an oxygen saturationlevel of hemoglobin within the volume of blood, the electromagneticradiation having at least one frequency component having an intensity,the intensity of the at least one frequency component of theelectromagnetic radiation being altered by interaction withoxyhemoglobin in the volume of intravascular blood thereby generating anoptical signal from the interaction of the at least one frequencycomponent of the electromagnetic radiation with the oxyhemoglobin in thevolume of intravascular blood, the optical signal having an intensity;and a light-to-frequency converter having an electromagnetic radiationsensor electrically connected to an electronic signal generator, saidelectromagnetic radiation sensor being optically coupled to saidelectromagnetic radiation source for reception of the optical signal andsaid electronic signal generator generating a periodic electrical signalresponsive to said electromagnetic radiation sensor, the periodicelectrical signal comprising a digital pulse train having pulses, thetiming of the pulses of the electrical signal corresponding to theintensity of the optical signal; wherein said electromagnetic radiationsensor comprises a photoresistor having a resistance, the resistancebeing variable and corresponding to an amount of electromagneticradiation illuminating said photoresistor; wherein said electronicsignal generator comprises an inverting circuit electrically connectedto a capacitor; wherein said capacitor charges and discharges over aperiod of time responsive to said inverting circuit and saidphotoresistor; and wherein the period of time in which said capacitorcharges and discharges corresponds to the resistance of saidphotoresistor.
 14. A pulse oximeter probe according to claim 13 whereinsaid inverting circuit has an input and an output and wherein saidphotoresistor is electrically connected in a feedback path between saidoutput and said input of said inverting circuit.
 15. A pulse oximeterprobe according to claim 13 further comprising an additional resistivedevice, wherein said inverting circuit has an input and an output, andwherein said photoresistor is electrically connected in series relationwith the additional resistive device in a feedback path between saidoutput and said input of said inverting circuit.
 16. A pulse oximeterprobe according to claim 13 wherein said inverting circuit has an inputand an output and wherein the digital pulse train of the periodicelectrical signal is generated at said output of said inverting circuit.17. A pulse oximeter probe according to claim 13 further comprising aninflatable cuff associated with said electromagnetic radiation sourceand wherein said electromagnetic radiation source is placed in opticalcommunication with a patient's skin associated with the volume ofintravascular blood.
 18. A diagnostic instrument probe for measuring aparameter of a cardiovascular system comprising: an electromagneticradiation source for illumination of a volume of intravascular bloodwith electromagnetic radiation, the electromagnetic radiation having atleast one frequency component having an intensity, the intensity of theat least one frequency component of the electromagnetic radiation beingaltered by interaction with the volume of intravascular blood therebygenerating an optical signal from the interaction of the at least onefrequency component of the electromagnetic radiation with the volume ofintravascular blood, the optical signal having an intensity; alight-to-frequency converter having an electromagnetic radiation sensorelectrically connected to an electronic signal generator, saidelectromagnetic radiation sensor being optically coupled to saidelectromagnetic radiation source for reception of the optical signal andsaid electronic signal generator generating a periodic electrical signalresponsive to said electromagnetic radiation sensor, the periodicelectrical signal comprising a digital pulse train having pulses, thetiming of the pulses of the electrical signal corresponding to theintensity of the optical signal; and a transmitter for wirelesslycommunicating to a remote receiver a signal corresponding to themeasured cardiovascular system parameter.
 19. A pulse oximeter probecomprising: an electromagnetic radiation source for illumination of avolume of intravascular blood with electromagnetic radiation, the volumeof intravascular blood having an oxygen saturation level of hemoglobinwithin the volume of blood, the electromagnetic radiation having atleast one frequency component having an intensity, the intensity of theat least one frequency component of the electromagnetic radiation beingaltered by interaction with oxyhemoglobin in the volume of intravascularblood thereby generating an optical signal from the interaction of theat least one frequency component of the electromagnetic radiation withthe oxyhemoglobin in the volume of intravascular blood, the opticalsignal having an intensity; a light-to-frequency converter having anelectromagnetic radiation sensor electrically connected to an electronicsignal generator, said electromagnetic radiation sensor being opticallycoupled to said electromagnetic radiation source for reception of theoptical signal and said electronic signal generator generating aperiodic electrical signal responsive to said electromagnetic radiationsensor, the periodic electrical signal comprising a digital pulse trainhaving pulses, the timing of the pulses of the electrical signalcorresponding to the intensity of the optical signal; and a transmitterfor wirelessly communicating to a remote receiver a signal correspondingto the measured oxygen saturation level of the hemoglobin within thevolume of blood.
 20. A probe for use in measuring a pulse rate of acardiovascular system comprising: an electromagnetic radiation sourcefor illumination of a volume of intravascular blood with electromagneticradiation, the electromagnetic radiation having at least one frequencycomponent having an intensity, the intensity of the at least onefrequency component of the electromagnetic radiation being altered byinteraction with the volume of intravascular blood thereby generating anoptical signal from the interaction of the at least one frequencycomponent of the electromagnetic radiation with the volume ofintravascular blood, the optical signal having an intensity; and alight-to frequency converter having an electromagnetic radiation sensorelectrically connected to an electronic signal generator, saidelectromagnetic radiation sensor being optically coupled to saidelectromagnetic radiation source for reception of the optical signal andsaid electronic signal generator generating a periodic electrical signalresponsive to said electromagnetic radiation sensor, the periodicelectrical signal comprising a digital pulse train having pulses, thetiming of the pulses of the electrical signal corresponding to theintensity of the optical signal.
 21. A probe for use in measuring apulse rate of a cardiovascular system according to claim 20 furthercomprising a transmitter for transmitting to a remote receiver a signalcorresponding to the pulse rate of the cardiovascular system.
 22. Aprobe for use in measuring a respiration rate of a cardiovascular systemcomprising: an electromagnetic radiation source for illumination of avolume of intravascular blood with electromagnetic radiation, theelectromagnetic radiation having at least one frequency component havingan intensity, the intensity of the at least one frequency component ofthe electromagnetic radiation being altered by interaction with thevolume of intravascular blood thereby generating an optical signal fromthe interaction of the at least one frequency component of theelectromagnetic radiation with the volume of intravascular blood, theoptical signal having an intensity; and a light-to-frequency converterhaving an electromagnetic radiation sensor electrically connected to anelectronic signal generator, said electromagnetic radiation sensor beingoptically coupled to said electromagnetic radiation source for receptionof the optical signal and said electronic signal generator generating aperiodic electrical signal responsive to said electromagnetic radiationsensor, the periodic electrical signal comprising a digital pulse trainhaving pulses, the timing of the pulses of the electrical signalcorresponding to the intensity of the optical signal.
 23. A probe foruse in measuring a respiration rate of a cardiovascular system accordingto claim 22, further comprising a transmitter for transmitting to aremote receiver a signal corresponding to the respiration rate of thecardiovascular system.
 24. A probe for use in measuring an intravascularvolume status of a cardiovascular system comprising: an electromagneticradiation source for illumination of a volume of intravascular bloodwith electromagnetic radiation, the electromagnetic radiation having atleast one frequency component having an intensity, the intensity of theat least one frequency component of the electromagnetic radiation beingaltered by interaction with the volume of intravascular blood therebygenerating an optical signal from the interaction of the at least onefrequency component of the electromagnetic radiation with the volume ofintravascular blood, the optical signal having an intensity; and alight-to-frequency converter having an electromagnetic radiation sensorelectrically connected to an electronic signal generator, saidelectromagnetic radiation sensor being optically coupled to saidelectromagnetic radiation source for reception of the optical signal andsaid electronic signal generator generating a periodic electrical signalresponsive to said electromagnetic radiation sensor, the periodicelectrical signal comprising a digital pulse train having pulses, thetiming of the pulses of the electrical signal corresponding to theintensity of the optical signal.
 25. A probe for use in meansuring anintravascular volume status of a cardiovascular system according toclaim 24, further comprising a transmitter for transmitting to a remotereceiver a signal corresponding to the intravascular volume status ofthe cardiovascular system.
 26. A method of producing a periodicelectrical signal indicative of a parameter of a cardiovascular systemcomprising: illuminating a volume of intravascular blood withelectromagnetic radiation, the electromagnetic radiation having at leastone frequency component having an intensity, the intensity of the atleast one frequency component of the electromagnetic radiation beingaltered by interaction with the volume of intravascular blood therebygenerating an optical signal from the interaction of the at least onefrequency component of the electromagnetic radiation with the volume ofintravascular blood, the optical signal having an intensity; opticallycoupling a light-to-frequency converter to receive the optical signal;and generating with the light-to-frequency converter the periodicelectrical signal comprising a digital pulse train having pulses, thetiming of the pulses of the electrical signal corresponding to aparameter of the optical signal.